Ion beam implanters are widely used in the process of doping of semiconductor wafers. An ion beam implanter generates an ion beam comprised of desired species of positively charged ions. The ion beam impinges upon an exposed surface of a semiconductor wafer workpiece thereby "doping" or implanting the workpiece surface with desired ions. Some ion beam implanters utilize serial implantation wherein a single semiconductor wafer workpiece is positioned on a support in an implantation chamber. The support is oriented such that the workpiece is in the ion beam beam line and the ion beam is repetitively scanned over the workpiece to implant a desired dosage of ions. When implantation is complete, the workpiece is removed from the support and another workpiece is positioned on the support.
Another type of ion beam implanter uses a rotating, translating disk-shaped support on which workpieces are mounted. A plurality of semiconductor workpieces are mounted on the disk-shaped support. The support is supported in an implantation chamber of an end or implantation station of the ion beam implanter. The rotation and translation of the support allows each of the plurality of workpieces to be exposed to the ion beam during a production run.
Faraday cages, which trap ions of the ion beam while blocking the escape of electrons from within the cage and excluding electrons which might accompany the beam, are universally used to measure ion beam current and thereby facilitate control of implantation dose. However, neutral atoms in the ion beam are not detected by the Faraday cage. If significant neutralization of the ion beam occurs, the Faraday cage ion beam current reading will give a false measure of the true ion implantation received by the workpieces.
Accuracy in quantity of ions implanted in the semiconductor wafer workpieces during the implantation process is of critical importance. The allowable tolerances on uniformity and total implantation dose in the manufacturing of semiconductor devices are now at the 1% level or lower in many applications. At these low tolerance levels, it is necessary to take into account the neutralization of the ions along the ion beam path. Neutralization of ions results from collisions of charged ions with residual atoms and electrons present in the interior region of the ion beam implanter along the beam path or beam line. Such neutralized ions have essentially the same energy as the charged ions and are essentially equivalent to them insofar as implantation dose is concerned.
Residual atoms in the ion beam implanter interior region and, particularly, residual gas atoms in the interior region result from at least three different sources. First, gas is injected into the interior region in connection with an ion beam neutralizer or electron shower. An ion beam neutralizer is disposed along the beam line and neutralizes the positively charged ions of the ion beam prior to implantation. If the positive charge on the ions is not neutralized prior to implantation of the wafers, the doped wafers will exhibit a net positive charge. Such a net positive charge on a wafer workpiece has undesirable characteristics. A neutralization gas is injected into the ion beam electron shower, collisions between the ion beam ions and the injected neutralization gas result in neutralized ions in the beam line. In certain ion beam implanters, neutralization gas associated with the ion beam neutralizer accounts for the greatest volume of residual gas in the ion beam implanter interior region. Typical neutralization gases include xenon (Xe) and Argon (Ar).
Accounting for the second greatest volume of residual gas in the ion beam implanter interior region in certain ion beam implanters is outgassing from photoresist material coated on the semiconductor wafer workpieces. In certain ion beam implanters, photoresist material outgassing accounts for the greatest volume of residual gas. As the ion beam impinges on the workpiece surfaces, the photoresist material is volatized or outgassed. Photoresist outgas is mainly comprised of hydrogen gas (H.sub.2), a variety of hydrocarbons, with a small amount of atmospheric nitrogen gas (N.sub.2) trapped by the photoresist.
A much smaller source of residual gas in the ion beam interior region results from source gas which escapes from a plasma chamber of the ion source. Source gases are injected into and are ionized within the plasma chamber. Ions escaping the plasma chamber through an opening or arc slit in a cover of the plasma chamber are accelerated along the ion beam beam line. A small amount of the source gas escapes through the arc slit and accounts for a low portion of the residual gas in the ion beam implanter interior region. Typical examples of source gases include arsine (AsH.sub.3), vaporized antimony (Sb), phosphine (H.sub.3 P), diborane (B.sub.2 H.sub.6), boron triflouride (BF.sub.3), vaporized gallium (Ga), vaporized indium (In), ammonia (NH.sub.3), hydrogen (H.sub.2) and Nitrogen (N.sub.2).
When the pressure in the interior region of the implanter along the beam line is low enough, the implanter species is essentially a singly charged positive ion selected by an analyzing magnet of the ion beam implanter. The analyzing magnet is positioned along the beam line and causes the ion beam to curve toward the implantation chamber. The strength and direction of the magnetic field of the analyzing magnet is set such that only ion species with a proper atomic weight are deflected at the proper radius of curvature to follow the desired beam line path to the implantation chamber. If, however, the pressure in the interior region of the ion beam implanter along the beam line is not low enough, a significant proportion of charged ions of the ion beam will undergo a change in their charge state through atomic collisions with the residual gas atoms, without undergoing a significant change in energy. In such circumstances, the ion beam striking the Faraday cage will contain a portion of neutral atoms. These neutralized atoms are the desired species and have the desired energies for implantation, thus, such neutralized atoms should be counted in a total flux of the ion beam. However, the Faraday cage is not capable of counting such neutralized atoms.
U.S. Pat. No. 4,539,217 to Farley, issued on Sep. 3, 1985, discloses a method and apparatus to compensate for neutralization of the ion beam in the implantation process. The Farley patent is assigned to the assignee of the present application and is fully incorporated by reference herein. The Farley patent utilizes the fact that the amount of ion beam neutralization is a function of the gas pressure in the interior region of the ion beam implanter along the ion beam beam line. Further, according to the Farley patent, the effective ion beam current, I.sup.T, is comprised of two components, the ion beam ionized singly positive charged current, I.sup.+, and the ion beam neutral current, I.sup.0. The effective ion beam current, I.sup.T, is a measure of current efficacious in the implantation of workpieces, regardless of charge of the implanted particles. Thus both the ionized ion beam current, I.sup.+, and the ion beam neutral current, I.sup.0, should be considered in determining the ion dosage a particular workpiece has received. The Farley patent assumes that the current measured by the Faraday cage, I.sup.f, is comprised solely of the ionized singly positive charged current, I.sup.+.
The second component of the true or effective ion beam current, I.sup.T, i.e., the neutral current, I.sup.0, is not measured by the Faraday cage. The atoms comprising the neutral current, I.sup.0, however, are just as effective in implanting the semiconductor wafer workpieces as are the ions comprising the ionized positive charged current, I.sup.+. Further, the greater the gas pressure in the ion implanter interior region, the greater the neutral current, I.sup.0, will be because of more collisions between ions and gas atoms and the smaller the ionized positive charged current, I.sup.+ will be. The Farley patent assumes that within a range of pressures encountered in the implantation process, the ion beam current measured by the Faraday cage I.sup.f, is a linear function of the pressure, P, of the ion implanter interior region.
The method disclosed in the Farley patent compensates for differential between the effective ion beam current, I.sup.T, and the ionized beam current, I.sup.+. The measurement of the ionized positive charged current, I.sup.+, and pressure, P, in the interior region of the ion implanter is used in an ion dosage control system to generate a calibration signal which compensates for the change in ions detected by the Faraday cage as the implanter interior region pressure varies. The calibration signal for a particular production run depends upon a selected calibration factor or "K" value.
The Farley patent method of controlling ion beam dose included the steps of:
1) measuring the ionized beam current, I.sup.+, incident on a wafer workpiece using a Faraday cage; PA1 2) measuring the gas pressure P within the implantation chamber; PA1 3) using a relationship to convert the ionized beam current, I.sup.+, and the pressure measurement P into a true or effective beam current, I.sup.T, and PA1 4) varying the dose of implantation as a function of the effective beam current, I.sup.T.
The true or effective ion beam current, I.sup.T, was then input to a microprocessor based implanter dose control system for use in accordance with known practices for monitoring and control of implantation dose.
According to the Farley patent, a linear equation used to convert the ionized ion beam current, I.sup.+, into an effective ion beam current, I.sup.T, is: EQU I.sup.T =I.sup.+ [1+KP]
Two modes of implanter operation were set forth in the Farley patent. In the first or fixed mode, a set of K values were estimated for different combinations of ion beam parameters and wafer workpiece parameters. The set of K values were stored in microprocessor memory and an appropriate value of K extracted from memory when the ion beam and workpiece characteristics were input to the microprocessor. In a second or dynamic mode of operation, a starting K value was selected, the K value was modified after each full rotation of the workpiece support. Upon each revolution of the workpiece support, the ionized beam current, I.sup.+, and the pressure P were measured and a value of K, called K.sub.j, was calculated (K.sub.j being the value of K for the jth rotation of the support). A moving average of the three most recent Kj's (K.sub.j, K.sub.j-1, and K.sub.j-2) was found and the moving average value, called K.sub.j.sup.A, was used to calculate a new effective beam current for the jth support revolution, called I.sub.j.sup.T.
In either mode of operation, an initial value of K must be provided to the dosage control system. Since K values were empirically estimated for different source gas/workpiece material combinations, a test implant matrix had to be generated for each source gas and each semiconductor wafer material. There was no assurance that any particular empirically determined K value was optimal or near optimal for the source gas/wafer material combination. Further, actual semiconductor wafers were used in testing to empirically determine K values. The testing resulted in the improper implantation of numerous wafers. Such semiconductor wafers have a significant cost per wafer and improper wafer implantation results in a significant scrap loss. Moreover, valuable production time is lost during testing runs in trying to find near acceptable K values for different ion beam parameters and wafer workpiece parameters.
What is needed is an effective ion beam dosage control apparatus for an ion implanter. What is also needed is an apparatus and process for efficiently determining optimal or near optimal K values which can be used to control ion beam dosage applied to workpieces. What is further needed is an apparatus and process for determining optimal K values without using actual semiconductor wafers.